Shortwave relay stations are transmitter sites used by international broadcasters to extend their coverage to areas that cannot be reached easily from their home state. E.g., the BBC operates an extensive net of relay stations. These days the programs are fed to the relay sites by satellite, cable/optical fiber or the Internet. Frequencies, transmitter power and antennas depend on the desired coverage. Some regional relays even operate in the medium wave or FM bands. Relay stations are also important to reach listeners in countries that practice radio jamming. Depending on the effect of the shortwave dead zone the target countries can jam the programs only locally, e.g. for bigger cities. For this purpose Radio Free Europe/Radio Liberty with studios in Munich, Germany operated a relay station in Portugal, in the extreme west of Europe, to reach then-communist Eastern Europe.
Variations in design
Two and only one broadcasting technology couples all of the components of a traditional shortwave relay station into one unit: the ALLISS module. For persons totally unfamiliar with the concepts of how shortwave relay stations operate this design may be the most understandable. The ALLISS module is a fully rotatable antenna system for high power shortwave radio broadcasting—it essentially is a self contained shortwave relay station. Most of the world's shortwave relay stations do not use this technology, due to its cost.
Planning and design
A traditional shortwave relay station—depending on how many transmitters and antennas that it will have—may take up to two years to plan. After planning is completed, it may take up to five years to construct the relay station. The historically long design and planning cycle for shortwave relay stations ended in the 1990s. Many advanced software planning tools became available. Choosing a series of sites for a relay station is about 100 times faster using Google Earth, for example. With the modern graphical version of Ioncap, simplified propagation studies can completed in less than a week for any chosen site. In some cases, existing relay stations can have their designs more or less duplicated, thus speeding up development time. However, there is one general exception to this: the ALLISS Module. From initial planning to deployment of ALLISS Modules may take a mere 1.5 years to 9 months depending on the number of modules deployed at one time in a particular sector of a country.
Operation
These are considered general operating parameters:
20 hours per day, but geopolitical reasons may dictate some stations run 24 hours per day
Generally 360 days per year, depending on the number of redundant transmitters and antennas
Relay Stations generally consume from 250 kilowatts to 10 megawatts
A single 100 kW SW transmitter consumes 225 kW RMS as a general rule
A single 300 kW SW transmitter consumes 625 kW RMS as a general rule
Modulator efficiency: Class-B modulators have about a 65% efficiency level, but digital modulators have about an 85% efficiency level as a general rule
Broadcast times and frequencies are under ITU regulation
generally to target areas that are more than 300 km from the transmitter site
most shortwave relay station target areas are 1500 km to 3500 km from the transmitter site
Mobile relay stations
The IEEEBook series "The History of International Broadcasting" describes mobile shortwave relay stations used by the German propaganda ministry during WWII, to avoid them being located by radio direction finding and bombed by the Allies. They consisted of a generator truck, transmitter truck and an antenna truck, and are thought to have had a radiated power of about 50 kW. Radio Industry Zagreb currently produces mobile shortwave transmitters.
The International broadcasting center of TDF is at Issoudun/Saint-Aoustrille. As of 2011, Issoudun is utilized by TDF for shortwave transmissions. The site uses 12 rotary ALLISS antennas fed by 12 transmitters of 500 kW each to transmit shortwave broadcasts by Radio France Internationale, along with other broadcast services.